ZnO/Cu(InGa)Se2 solar cells prepared by vapor phase Zn doping

ABSTRACT

A process for making a thin film ZnO/Cu(InGa)Se 2  solar cell without depositing a buffer layer and by Zn doping from a vapor phase, comprising: depositing Cu(InGa)Se 2  layer on a metal back contact deposited on a glass substrate; heating the Cu(InGa)Se 2  layer on the metal back contact on the glass substrate to a temperature range between about 100° C. to about 250° C.; subjecting the heated layer of Cu(InGa)Se 2  to an evaporant species from a Zn compound; and sputter depositing ZnO on the Zn compound evaporant species treated layer of Cu(InGa)Se 2 .

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. DE-AC3699GO10093 between the United States Department of Energy andthe National Renewable Energy Laboratory, a division of the MidwestResearch Institute.

TECHNICAL FIELD

The present invention relates to Zinc oxide/Copper Indium GalliumSelenium Disulfide solar cells characterized by conversion efficienciesof >11%. The cells are prepared by subjecting Copper Indium GalliumDisulfide (CuInGaSe₂) or CIGS thin films to evaporant species from zincacetate dihydrate at varying substrate temperatures, and sputterdepositing ZnO thereon to provide a transparent electrode for the cell.The innovation of the invention constitutes a basis for in-line junctionfabrication processes for preparing CIGS thin films that do not requirethe use of a Cd compound and requires no buffer layers.

BACKGROUND ART

Processes for manufacture of high light to electrical energy conversionefficiency thin film photovoltaic cells are known to utilize a firstlayer of copper indium diselenide in heterojunction with one or morelayers of cadmium sulfide.

For example, U.S. Pat. No. 4,335,266 discloses a method for forming acopper indium diselenide layer in two distinct regions, wherein thefirst region contains an excess of copper and the second region iscopper deficient. Diffusion between the two layers achieves a uniformcopper indium diselenide structure to reduce the formation of purecopper nodules near the copper indium diselenide surface where thecadmium sulfide layer is to be deposited. However, despite theimprovements in the copper indium diselenide layer, it has still beenfound necessary to deposit a cadmium sulfide layer to achieve highefficiency.

“While various improvements have been made in the manufacture of copperindium diselenide CdS cells, several complications remain. For example,chemical bath deposition of cadmium sulfide is used to produce thehighest efficiency devices. However, this step involves a slow wetchemical step inconsistent with an otherwise in-line dry fabricationprocess. Moreover, cadmium and thiourea are highly toxic materials whichescalate manufacturing costs as a result of the handling and disposal ofthese hazardous waste materals”.

Several attempts to avoid handling complications inherent in the use ofCdS are disclosed in “A ZnO/p-CuInSe₂ Thin Film Solar Cell PreparedEntirely by Spray Pyrolysis,” M. S. Tomar and F. J. Garcia, Thin SolidsFilms, 90 (1982). p. 419–423; and “Chemical Vapor Deposited CopperIndium Diselenide Thin Film Materials Research” Final Report, March1984, SERI/STR-211-2247. Although these publications disclose copperindium diselenide/zinc oxide heterojunction formation using zinc oxidespray pyrolysis or ion beam sputtering respectively, neither methodresults in an efficiency of greater than 2–3%. Accordingly, thesepublications do not disclose a commercially viable method for thereplacement of CdS with zinc oxide in a thin film copper indiumdiselenide heterojunction cell.

U.S. Pat. No. 4,612,411, describes the preparation of a thin filmheterojunction photovoltaic cell formed from copper indium diselenide,as a first semiconductor layer, and the formation of a two layer, zincoxide semiconductor in heterojunction with the copper indium diselenide.The first of the two zinc oxide layers comprises a relatively thin layer(100–2000 angstroms) of bigh resistivity zinc oxide and the secondcomprises a relatively thick (10,000 angstroms) zinc oxide layer dopedto exhibit low resistivity.

U.S. Pat. No. 5,474,939, produces a higher efficiency non-CdS cellthrough the application of a wet chemical deposition zinc hydroxideprecipitation step. This process involves the use of a metal backcontact having a first p-type semiconductor film of chemical vapordeposition (“CVD”) copper indium diselenide and a second transparentn-type semiconductor film of CVD zinc oxide on the copper indiumdiselenide and a thin interfacial film of transparent insulating zincoxide, between the p-type copper indium diselenide film and the n-typezinc oxide. The interfacial zinc oxide film is prepared by chemicaldeposition of zinc hydroxide on the copper indium diselenide from a zincsalt solution and complexing agents comprising ammonium hydroxide ortriethanolamine, to form a zinc ammonium solution complex, and annealingthe deposit to covert the zinc hydroxide into the zinc oxide. While thispatent uses a wet chemical deposition step of zinc hydroxide precipitatefrom solution to generate a thin interfacial zinc oxide layer, thedevices prepared by direct deposition of a zinc oxide layer, the devicesprepared by direct deposition of a zinc oxide layer on copper indiumdiselenide films are only 2–4% conversion efficient in spite ofutilizing films capable of producing 15–17% cells.

Accordingly, there is a need in the art to obtain copper indium galliumdiselenide thin film photovoltaic cells characterized by high conversionefficiency without incurring the disadvantages of: utilzing a CdS layervia wet chemistry; utilizing slow, batch processes; employing the highlytoxic material of Cd, which escalates manufacturing costs as a result ofhandling and disposal of this hazardous waste; incurring a reduction incollected current; and forming junctions at 60–80° C.

DISCLOSURE OF THE INVENTION

One object of the present invention is to provide an improved processfor making CuInGaSe₂ thin film solar cells characterized by conversionefficiencies greater than about 11%, and yet avoid the need to deposit abuffer layer.

Another object of the present invention is to provide a process forpreparing CuInGaSe₂ thin film solar cells characterized by conversionefficiencies greater than about 11% by subjecting CuInGaSe₂ thin filmsto an evaporant species from zinc acetate dihydrate to dope the surfaceregion n-type.

Other objects and advantages of the invention will become apparent fromthe drawings and detailed description of the preferred embodiment of theinvention hereafter described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing current density versus voltage for a CuInGaSe₂(CIGS) thin film or diode that was not subjected to an evaporant speciesof zinc acetate dihydrate and on which ZnO was sputter deposited.

FIG. 2 is a graph showing current density versus voltage for a CIGS thinfilm subjected to an evaporant species of zinc acetate dihydrate at asubstrate temperature or Tg of 150° C., followed by sputter depositingZnO.

FIG. 3 is a graph showing current density versus voltage for a CIGS thinfilm subjected to an evaporant species of zinc acetate dihydrate at asubstrate temperature or Tg of 200° C., followed by sputter depositingZnO.

FIG. 4 is a graph of current density versus voltage for a CIGS thin filmsubjected to an evaporant species of zinc acetate dihydrate at asubstrate temperature or Tg of 250° C., followed by sputter depositingZnO.

FIG. 5 is a CIGS film on Mo/glass cut into three 2″×1″ samples (A1, A2and A3) and a blank, wherein one CIGS sample and a blank is used in eachrun of the invention process, and the blank is saved as the bottomthird.

FIGS. 6A THROUGH 6G are graphs showing dark and light I-V curves for the7 cells shown on a substrate 1×1.5 in². The film were subjected toevaporant species from zinc acetate dihydrate at 150° C., followed bysputter deposition of ZnO. Each figure is for one cell, and the twolines are for dark and light I-V.

FIG. 6H shows a quantum efficiency curve of relative external quantumefficiency (QE) versus wavelength for the ZnO/CIGS cells of FIG. 6prepared by the Zn vapor phase doping of the invention process.

FIGS. 7A through 7G are graphs showing two sets of I-V curves for astring of seven cells adjacent to each other on a single substrate, onefor dark and the other for light, 1×1.5 in². The broken lines are I-Vcurves taken after the cells are annealed in air at 200° C. for 2minutes. The CIGS absorber was exposed to evaporant zinc acetatedehydrate at a substrate temperature of 100° C., and heated to 200° C.for 30 minutes.

FIGS. 8A through 8F are graphs showing I-V curves of the distribution ofefficiency for multiple cells on a single substrate, wherein the CIGSsample was exposed to evaporant species from zinc acetate dehydrate at asubstrate temperature of 200° C. and the ZnO layer was deposited bysputtering. There are many cells with an efficiency of 11%, and the bestcell had an efficiency of 12.6%.

FIG. 9 is a graph showing I-V curves using essentially the same criteriaas FIGS. 8A through 8F, but has an efficiency of 13.1%, an open currentvoltage of 0.627V, a current density of 32.8 mA/cm², and a fill factorof 64%.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION

It is known that Cd and Zn have a profound influence in creatingefficient photovoltaic junctions in CuInSe₂ (CIS) thin films since 11%cells can be fabricated in CIS thin films with Cd and Zn. On the otherhand, with CuInGaSe₂ thin films, 15.7% cells have been produced using Cdand 14.2% cells have been produced using Zn. By the use of a non-aqueousmethod to diffuse Zn a cell with 13.5% efficiency is produced.

With demand mounting in the thin film photovoltaic industry, CIS cellscan benefit greatly from a simple, quick, in-line process for doping andjunction-formation. The present invention process meets this demand inthat hot CIGS is cooled to about 200° C., and made to enter a dopingzone wherein it is exposed to an evaporant species of ZnAc₂.2H₂O or itsequivalents, and exits to the transparent conducting oxide (TCO)deposition of ZnO by sputtering.

Equivalents of zinc acetate dihydrate in the context of the inventionprocess are zinc chloride, zinc iodide, and zinc bromide. Further,cadmium chloride, cadmium iodide, and cadmium bromide will produceeffects similar to that of the aforementioned zinc compounds.

In the vapor phase and n-type doping and junction formation of CuInGaSe₂thin film CIGS cells of the invention, the CIGS thin films weresubjected to an evaporant species from zinc acetate dihydrate at varioussubstrate temperatures. The samples were etched in acetic acid and solarcells were completed by the sputter deposition of ZnO to obtain cells ashigh as ˜11% efficiency for the case of 200° C. and this is a markedimprovement compared to the 1.8% efficiency for the case of no-vaporphase ZnO sputter deposition treatment. As a consequence, a non-Cd,in-line junction fabrication process for CIGS is now available for thephotovoltaic industry.

EXAMPLE

A co-evaporated CIGS film was divided into four parts, 2″×1″ in lengthand loaded into a chamber along with a Mo/glass substrate. Zinc acetatedihydrate solid was loaded in a Mo boat. A thermocouple touching theback side of the CIGS substrate was used to control the temperature(Tglass).

The zinc acetate dihydrate source temperature was raised slowly and thecharge was evaporated slowly, while the substrate is brought to thedesired temperature before the evaporation is started. The substrate wasexposed during the entire time. The exposure time was not controlled,and the evaporation lasted for about 2 to about 4 minutes. The sampleswere removed and etched in acetic acid in an amount of about 50% byvolume in water to remove the ZnO deposit. Sputtered ZnO was depositednext in MRC.

The graph of FIG. 1 shows a weak diode in the case of no treatment inaccordance with the invention process.

In FIG. 2 where Tg=150° C., and was treated in accordance with theinvention process, the sample had a dark brown coating, and showeddeposition of ˜30 nm ZnO or Zn(OH)₂ film. For Tg=200° C. and 250° C., asshown respectively in the graphs of FIGS. 3 and 4, no clear change incolor was observed. All the samples were etched in acetic acid.

In the case where the Tg=150° C., a clear improvement is observed, andthe Jsc is in the expected range. Further, there is >60% fill factor,and this is an indication that the process is satisfactory. The Voc isnearly 0.5V, and this is again in the range for non-CdS cells as >9%efficiency is obtained.

For Tg=200° C., the Voc is 0.562V, and the fill factor is 0.63. Theseare satisfactory numbers. The typical Voc's for aqueous Cd photoelectrictreated cells are about 0.6V, and fill factors are about 0.65–0.7. Thediode formed functions satisfactorily in darkness and light. A reactionat the surface of the CIGS is raising the potential barrier (spike) andlimiting the current flow; however the absorber is likely compensated,as (p is reduced by Zn donors). The cross over is indicative of defectschanging occupancy under illumination (trapping).

For Tg=250° C., as shown in FIG. 4, these problems become worse. Theabsorber is now semi-insulating due to excessive compensation. There isa similar trend with CdS/CIGS as a function of post heating.

FIG. 5 is a CIGS film on Mo/glass cut into three 2″×1″ samples (A1, A2and A3) and a blank, wherein one CIGS sample and a blank is used in eachrun of the invention process, and the blank is saved as the bottomthird.

FIG. 6 is a graph showing dark and light I-V curves for the 7 cellsshown on a substrate 1×1.5 in² shown in FIGS. 6A thru 6G. The film weresubjected to evaporant species from zinc acetate dihydrate at 150° C.,followed by sputter deposition of ZnO. Each figure is for one cell, andthe two lines are for dark and light I-V.

FIG. 6H shows a quantum efficiency curve of relative external quantumefficiency (QE) versus wavelength for the ZnO/CIGS cells of FIG. 6prepared by the Zn vapor phase doping of the present invention process.

FIGS. 7A through 7G are graphs showing two sets of I-V curves for astring of seven for cells adjacent to each other on a single substrate,one for dark and the other for light, 1×1.5 in². The broken lines areI-V curves taken after the cells are annealed in air at 200° C. for 2minutes. The CIGS absorber was exposed to evaporant zinc acetatedehydrate at a substrate temperature of 100° C., and heated to 200° C.for 30 minutes. The data for the two sets of curves are shown below:

Area Voc Jsc Isc FF Eff Vmp Imp Pmax Rso Rsho Cell Process (cm²) (V)(mA/cm²) (mA) (%) (%) (V) (mA) (mW) (Ω · cm) (Ω · cm) 1 As dep 0.4300.490 −30.32 −13.039 65.61 9.751 0.369 −26.45 9.751 2.718 673.9 2 As dep0.430 0.487 −29.43 −12.655 65.49 9.385 0.365 −25.72 9.385 2.559 717.5 3As dep 0.430 0.479 −29.55 −12.708 64.75 9.161 0.359 −25.55 9.161 2.149523.1 4 As dep 0.430 0.474 −29.90 −12.859 64.67 9.161 0.353 −25.93 9.1612.752 622.0 5 As dep 0.430 0.465 −30.47 −13.100 65.17 9.230 0.350 −26.359.230 1.920 566.5 6 As dep 0.430 0.460 −31.46 −13.529 64.39 9.320 0.345−27.02 9.320 2.411 562.4 7 As dep 0.430 0.458 −30.85 −13.265 63.61 8.9860.337 −26.69 8.986 2.044 539.8 Area Voc Jsc Isc FF Eff Vmp Imp Pmax RsoRsh Cell Process (cm²) (V) (mA/cm²) (mA) (%) (%) (V) (mA) (mW) (Ω · cm)(Ω · cm) 1 As dep 0.430 0.490 −30.32 −13.039 65.61 9.751 0.369 −26.459.751 2.718 673.9 2 As dep 0.430 0.487 −29.43 −12.655 65.49 9.385 0.365−25.72 9.385 2.559 717.5 3 As dep 0.430 0.479 −29.55 −12.708 64.75 9.1610.359 −25.55 9.161 2.149 523.1 4 As dep 0.430 0.474 −29.90 −12.859 64.679.161 0.353 −25.93 9.161 2.752 622.0 5 As dep 0.430 0.465 −30.47 −13.10065.17 9.230 0.350 −26.35 9.230 1.920 566.5 6 As dep 0.430 0.460 −31.46−13.529 64.39 9.320 0.345 −27.02 9.320 2.411 562.4 7 As dep 0.430 0.458−30.85 −13.265 63.61 8.986 0.337 −26.69 8.986 2.044 539.8 1 02m200 0.4300.513 −29.96 −12.883 67.27 10.349 0.395 −26.20 10.349 2.441 402.6 202m200 0.430 0.508 −29.08 −12.504 65.86 9.732 0.383 −25.39 9.732 2.498507.0 3 02m200 0.430 0.501 −29.20 −12.555 63.24 9.246 0.373 −24.76 9.2462.717 349.8 4 02m200 0.430 0.496 −29.51 −12.689 60.57 8.856 0.369 −24.028.856 2.756 319.5 5 02m200 0.430 0.492 −30.00 −12.902 60.05 8.856 0.363−24.37 8.856 2.742 271.5 6 02m200 0.430 0.492 −31.06 −13.355 61.61 9.4240.363 −25.94 9.424 2.582 331.9 7 02m200 0.430 0.491 −30.22 −12.993 62.369.255 0.369 −25.11 9.255 2.482 283.4

FIGS. 8A through 8F are graphs showing I-V curves of the distribution ofefficiency for multiple cells on a single substrate, wherein the CIGSsample was exposed to evaporant species from zinc acetate dehydrate at asubstrate temperature of 200° C. and the ZnO layer was deposited bysputtering. There are many cells with an efficiency of 11%, and the bestcell had an efficiency of 12.6%. The data for these two sets of curvesare shown below:

Area Voc Jsc Isc FF Eff Vmp Imp Pmax Rso Rsho Cell Process (cm²) (V)(mA/cm²) (mA) (%) (%) (V) (mA) (mW) (Ω · cm) (Ω · cm) 1 As dep 0.4300.516 −30.62 −13.166 57.59 9.103 0.404 −22.52 9.103 2.431 71.79 2 As dep0.430 0.532 −31.34 −13.475 67.76 11.293 0.423 −26.69 11.293 1.962 242.873 As dep 0.430 0.516 −30.84 −13.262 50.33 8.005 0.384 −20.85 8.005 2.62550.65 4 As dep 0.430 0.540 −30.97 −13.316 70.91 11.862 0.433 −27.3911.862 1.814 805.79 5 As dep 0.430 0.542 −30.04 −12.916 71.08 11.5760.433 −26.73 11.576 1.703 966.72 6 As dep 0.430 0.548 −30.19 −12.98371.47 11.818 0.440 −26.84 11.818 1.966 1062.24 7 As dep 0.430 0.563−30.98 −13.323 72.54 12.647 0.453 −27.91 12.647 1.620 1119.81

FIG. 9 is a graph showing I-V curves using essentially the same criteriaas FIGS. 8A through 8F, but has an efficiency of 13.1%, an open currentvoltage of 0.627V, a current density of 32.8 mA/cm², and a fill factorof 64%.

As a result of these tests, it seems apparent from these cells thatseveral issues are significant:

1. It appears useful to characterize the ZnO (Zn(OH₂) deposit by XRD;

2. It may be useful to utilize a high efficiency, well characterizedabsorber;

3. It may be useful to use a shutter and control the exposure time;

4. It may be useful to evaporate zinc acetate on CIGS that is notheated, and post annealing appears useful to induce diffusion anddoping;

5. It may be useful to either keep the ZnO on or remove it; and

6. It may be useful to wash the CIGS in water or dilute NH₄OH beforedeposition to remove surface alkali impurities.

That a simple evaporation process of a Zn species, that may be zincacetate dihydrate for vapor phase, extrinsic n-type doping of CIGS thinfilms can achieve at least 11% conversion efficiency in CIGS cells isapparent from the J-V data which follows.

Although certain preferred embodiments have been described and shown inthe examples and the accompanying drawings, it is to be understood thatthese embodiments are merely illustrative of and not restricted to theinvention scope and various modifications and changes may occur to thoseof ordinary skill in the art without departing from the spirit and scopeof the invention, which is described hereinafter in the affixed claims.

1. A process for making a thin film ZnO/Cu(InGa)Se₂ solar cell withoutdepositing a buffer layer and by Zn doping from a vapor phase,comprising: a) depositing Cu(InGa)Se₂ layer on a metal back contactdeposited on a glass substrate; b) heating the Cu(InGa)Se₂ layer on saidmetal back contact on said glass substrate to a temperature rangebetween about 100° C. to about 250° C.; c) subjecting the heated layerof Cu(InGa)Se₂ to an evaporant species from Zn acetate dihydrate to dopethe Cu(InGa)Se₂ with Zn and form a ZnO deposit and etching with aceticacid in an amount of 50% by volume in water to remove the ZnO deposit;and d) sputter depositing ZnO on the Zn acetate dihydrate evaporantspecies treated layer of Cu(InGa)Se₂.
 2. The process of claim 1 whereinsaid metal back contact is Mo.
 3. The process of claim 2 wherein in stepc) the heated layer of Cu(InGa)Se₂ is subjected to said evaporantspecies from said zinc acetate dihydrate under a vacuum.
 4. The processof claim 3 wherein the substrate temperature is about 100° C. duringsaid heating.
 5. The process of claim 4 wherein, prior to sputterdepositing ZnO in step d) an annealing step is performed at atemperature range from about 150° C. to about 200° C.
 6. The process ofclaim 3 wherein the substrate temperature is about 150° C. during saidheating.
 7. The process of claim 6 wherein, prior to sputter depositingZnO in step d) an annealing step is performed at a temperature rangefrom about 150° C. to about 200° C.
 8. The process of claim 3 whereinthe substrate temperature is about 200° C. during said heating.
 9. Theprocess of claim 8 wherein, prior to sputter depositing ZnO in step d)an annealing step is performed at a temperature range from about 150° C.to about 200° C.
 10. The process of claim 3 wherein the substratetemperature is between 200° C. and 250° C. during said heating.
 11. Theprocess of claim 10 wherein, prior to sputter depositing ZnO in step d)an annealing step is preformed at a temperature range from about 150° C.to about 200° C.